A dual-spin satellite is a stabilized, or unstabilized, orbiting device having a spinning portion or rotor, and a despun portion or platform. However, other configurations are possible wherein only an antenna is despun. The two portions are coupled together by a despin bearing and power transfer assembly (BAPTA) whose axis is parallel to the spin vector. The platform or antenna is usually pointed at a predetermined position on the earth, or at another satellite, or the like. Accurate control of the pointing position of the platform is critical to satellite operation.
The pointing position of the dual-spin satellite may be controlled by controlling the relative rate and relative position or phase of the payload platform with respect to the spinning rotor. Prior art satellite pointing control systems include an open-loop system which provides for operator control of motor torque commands applied to an orbiting satellite from an earth station. The operator manipulated the rotation rate of the payload platform with respect to the earth to a low level, after which a position control loop controls the pointing position of the platform.
Another system provides for a position control loop which is utilized when the pointing direction of the platform is within a predetermined angular extent (dead band), while fixed torque signals are applied outside of the dead band. The angular extent of the dead band is based upon the relative rotational rate between the payload platform and rotor. A further control system is described in the commonly owned U.S. Pat. No. 4,437,047, issued on Mar. 13, 1984 to John W. Smay, which is hereby incorporated by reference. This control system utilizes a closed-loop feedback system which employs both position and rotation rate information to control the pointing position of a payload platform of a dual-spin satellite. In this system, index pulses and earth pulses are utilized to generate error signals which in turn produce position torque command signals.
Another control system utilizes a phase locked loop which phase locks an index pulse train to an inertial sensor pulse train that indicates a known position. For multiple index pulses, this system can lock an inertial pulse to any index pulse. The system allows for manual stepping to any desired pulse which provides the correct pointing direction. One system which utilizes an analog phase locked loop circuit to provide an inertial reference for platform pointing control is described in "Attitude and Payload Control System for the Leasat Naval Communications Satellite", by Loren I. Slafer, presented at the Annual Rocky Mountain Guidance and Control Conference, 1982, American Astronauticl Society, AAS 82-007. This publication is hereby incorporated by reference.
It is a principle object of the present invention to provide a platform pointing controller and method for a dual-spin satellite which achieves high pointing control accuracy without requiring an RF beacon reference.
It is another object of the present invention to provide a precision platform pointing controller which is capable of transient free switching between earth and sun inertial attitude references.
It is an additional object of the present invention to provide a precision platform pointing controller which utilizes existing sensors and actuators mounted to the spinning portion of the spacecraft without requiring a despun platform mounted inertial sensor.
It is a further object of the present invention to provide a precision platform pointing controller which employs digital error detection and processing utilizing state estimator/controller algorithms.
It is yet another object of the present invention to provide a precision platform pointing controller which utilizes an onboard firmware driven microprocessor to substantially improve overall system performance in comparison to prior controller techniques.
It is still another object of the present invention to provide a precision pointing controller which implements despin and nutation control functions in microprocessor firmware to realize substantial hardware savings and to substantially improve design flexibility, as well as to achieve a more precise implementation of these control functions.
It is still a further object of the present invention to provide a precision pointing controller which utilizes firmware to compensate for disturbances due to thruster firings and nutational movement, and firmware to correct for longitude drift and orbit eccentricity when a sun sensor is being used as the inertial attitude reference.